Preheat an oven to 350°F.
Rinse the chicken and pat dry. Rub the chicken inside and out with the salt and pepper. Using your fingers and starting at the cavity, gently separate the skin from the breast meat, reaching as far back toward the neck and as close to the thighs as possible to create a pocket. Be careful not to tear the skin. Slip the garlic slices between the skin and the meat, spreading them evenly over the breast. Place the chicken, breast side up, on a rack in a roasting pan. Add the stock to the pan.
Roast, basting occasionally, until an instant-read thermometer inserted into the thigh, away from the bone, registers 170°F, or until the juices run clear when a knife is inserted into the thigh joint, about 1 1/4 hours. Transfer to a carving board and let rest for 10 minutes before carving.
Meanwhile, begin making the salad: In a small fry pan over low heat, toast the walnuts, shaking the pan often, until a nut piece is golden brown when cut in half, about 10 minutes. Transfer to a plate and set aside.
In a large bowl, whisk together the walnut oil, vinegar, shallot, salt and pepper. Tear the lettuce leaves into bite-size pieces and add them to the bowl. Using tongs, toss gently to coat the lettuce well with the dressing.
Arrange the salad on a platter and sprinkle with half of the toasted walnuts.
Carve the chicken, slicing the breast meat and separating the thighs from the legs. Arrange the thighs, legs, wings and the sliced breast meat on top of the salad, including any bits of garlic that slipped from beneath the skin while carving. Sprinkle with the remaining walnuts and serve warm.